Rubber-reinforced vinyl aromatic polymers

ABSTRACT

Rubber-reinforced vinyl aromatic polymers having a strictly bimodal morphology, comprising from 55 to 90% by weight of a rigid polymeric matrix and from 10 to 45% by weight of a rubbery phase dispersed inside said rigid polymeric matrix, in the form of grafted and occluded particles and wherein said rubber particles consist of from 60 to 99% by weight of particles with a capsule or “core-shell” morphology and from 1 to 40% by weight of particles with a “salami” morphology, said percentages being measured on the basis of the weight of the rubber particles only.

This application is a divisional of application Ser. No. 10/561,266,filed Nov. 17, 2006, now pending; which is a 371 of PCT/EP04/07296 filedJul. 2, 2004, both incorporated herein by reference.

The present invention relates to rubber-reinforced vinyl aromaticpolymers.

More specifically, the present invention relates to compositionscomprising a rigid matrix consisting of vinyl aromatic polymers orcopolymers and a rubbery phase dispersed inside the matrix in the formof particles with a strictly bimodal distribution or morphology. Theterm “strictly bimodal distribution or morphology” as used in thepresent description and claims, indicates a series of rubber particles,randomly dispersed inside a rigid polymeric matrix, in which saidparticles have a bimodal morphology exclusively represented by a firstclass of particles (prevalent modal class) with a capsule or“core-shell” structure, having an average volume dimension ranging from0.15 to 0.25 μm and a second class of particles (subvalent modal class)with a so-called “salami” structure, having an average volume dimensionranging from 1 to 5 μm and the complete absence of particles with anintermediate structure or dimension between said two classes.

It is well known that the physico-chemical characteristics andmechanical properties of vinyl aromatic polymers reinforced with rubber,in particular of high impact polystyrene (HIPS), depend on severalfactors, among which the dimensions of the rubbery particles grafted onthe polymeric matrix and cross-linked.

It is also known that certain properties, such as the impact resistanceand gloss, in particular in HIPS, are influenced, in an opposite manner,by the average dimension and distribution of the diameters of therubbery particles, for a certain rubber concentration. In particular,the “large” particles increase the impact resistance of the material tothe detriment of the gloss, whereas the “small” particles reduce thetoughness but make the material extremely glossy.

Methods have been proposed in literature for obtaining rubber-reinforcedvinyl aromatic polymers, for example rubber-reinforced polystyrenes,having a good gloss combined, at the same time, with a good impactresistance. One of these methods, for example, envisages the addition tothe polymeric matrix of a limited number of “large” particles to amajority of “small” rubbery particles already present. The productsobtained are generically defined as high impact vinyl aromatic polymerswith a bimodal particle size distribution.

In the case of HIPS, this combination leads to a product with a synergyin the impact resistance combined with an excellent gloss.

The U.S. Pat. No. 4,153,645, for example, describes a HIPS with anenhanced property balance obtained by mechanically mixing(melt-blending) 50-85% by weight of a high impact polystyrene containingsmall rubbery particles (with an average diameter of about 0.2-0.9 μm)with 15-50% by weight of a high impact polystyrene containing largerrubbery particles (average diameter of about 2-5 μm). According to thispatent, the final product obtained by mixing the two HIPS, has impactand flexural resistance values higher than those expected by applyingthe blend rule, without any decrease in the other physical properties.

Using the same type of process (melt-blending), U.S. Pat. No. 4,493,922describes a HIPS with a bimodal morphology consisting of 60-95% byweight of “capsule” particles having a diameter of between 0.2 and 0.6μm and 40-5% by weight of particles with a “cell” and/or “coil”morphology, with a diameter ranging from 2 to 8 μm.

The U.S. Pat. No. 4,221,883; U.S. Pat. No. 4,334,039; EP 96,447; U.S.Pat. No. 4,254,236; EP 15,752 and international patent applicationsWO98/52985 and WO99/09080 describe the so-called “split-feedpolymerization” process for producing HIPS with a bimodal morphologywhich allows an improvement in the gloss/impact balance. According tothis process, the prevalent modal class of small particles is producedin two thirds of a pre-polymerization reactor, by feeding a dissolutionin styrene of a low viscosity polybutadiene rubber or a block copolymerhaving a suitable composition. A second styrene dissolution of a highviscosity polybutadiene rubber is feed in the remaining third of thereactor. The high viscosity polybutadiene, when in contact with thepreviously formed pre-polymer, undergoes a rapid phase inversion,forming large particles, poorly grafted and which cannot be easilymodulated as far as the dimension is concerned.

U.S. Pat. No. 5,240,993 describes a method (“parallel polymerization”)for the preparation of impact resistance vinyl aromatic polymers,characterized by a bimodal distribution of the rubbery phase, accordingto a continuous mass process, using two plug flow reactors situated inparallel. A first pre-polymer containing a rubbery phase with smallparticles is prepared in one of the two reactors, whereas a secondpre-polymer, containing a rubbery phase with large particles, isprepared in the other reactor. The polymeric streams are mixed at theoutlet of the two reactors and the polymerization is completed in athird reactor, again of the plug flow type, called finishing reactor.

WO97/39040 describes a simplified version of this process, according towhich, large particles are produced in the first half of apre-polymerization reactor by feeding a suitable styrene solution of ahigh viscosity rubber, under such conditions as to guarantee a goodgrafting efficiency and an accurate dimensional control. Thelargeparticle pre-polymer is mixed in the second half of the samereactor, in suitable proportions, with a second pre-polymer having smallparticles, previously produced in a reactor placed in series with thefirst.

One of the draw-backs of the above processes is that they require:

-   In the case of “melt blending”, the use of a compounding step with a    consequent increase in the production costs, or the preparation of    HIPS components which cannot be easily sold as such.-   2. In the case of “parallel polymerization” or “split-feed    polymerization”, the development and construction of industrial    plants having a much more complex configuration (pre-polymerization    reactors in parallel, delayed feedings of rubber dissolutions,    reactors with partitioning septa) and equipped with much more    sophisticated control systems with respect to the standard plants    with polymerization reactors in series, used for producing    conventional HIPS.

In addition to systems for the preparation of HIPS with a bimodaldistribution of the reinforcing rubber particles, through the mixing ofpre-formed products, alternative “chemical” methods have been proposed,which allow these particular morphologies to be obtained by operating onthe formulations of the reaction feeds and using the same productionconfigurations adopted for the traditional HIPS.

European patent 418,042, for example, describes a method for producingrubber-reinforced vinyl aromatic polymers, in which the particles have a“generally bimodal” distribution or a broader distribution including, inaddition to the small (0.1-0.8 μm) prevalent modal class and the large(2-6 μm) subvalent modal class, also a third particle class having anintermediate dimension (0.8-2.0 μm). This distribution is obtained witha medium cis polybutadiene characterized by a bimodal distribution ofthe molecular weights and sold under the name of ASAPRENE 760 A.

European patent 731,016, similarly, describes the production of HIPSwith a bimodal morphology using, in a conventional configuration ofreactors, an elastomeric phase (dissolved in styrene) consisting of amedium cis and low viscosity polybutadiene and a high cis and highviscosity polybutadiene.

European patent 726,280 describes the production of HIPS with a bimodalmorphology by introducing suitable concentrations of stable nitroxylradicals during the HIPS polymerization step, with a conventionalreactor configuration and with a high cis polybutadiene rubber.

International patent application WO03/033559, similarly, describes HIPSwith a pseudo-bimodal morphology which can be obtained by introducingsuitable concentrations of functionalized nano-composite materials intothe HIPS polymerization with a conventional reactor configuration. Thefunction of the nano-composite material is to transform part of thelarge rubbery particles into small rubbery particles.

The methods proposed in all these patents, however, have at least thedrawback of not providing a “strictly bimodal” morphology of the rubberparticles but only “generally bimodal” or simply “broadened”.

Finally, European patent 620,236 proposes a method for obtaining HIPSwith a “strictly bimodal” morphology. According to this method, a smallamount of HIPS with large particles is dissolved in styrene togetherwith the polybutadiene rubber or styrene-butadiene block copolymernecessary for producing the prevalent modal class of small particles.The solution obtained is polymerized with a conventional plantconfiguration. During the whole polymerization the cross-linked rubberyparticles of the preformed HIPS do not undergo retro-inversion but keeptheir structure and dimension, whereas the polybutadiene rubber orstyrenebutadiene copolymer form small particles with a correspondingstructure and dimensions.

The basic limit of the technical solution proposed in this patent isrepresented by the highest percentage of preformed HIPS which can bedissolved in styrene together with the rubber (lower than 5%).

The Applicant has now found new rubber-reinforced vinyl aromaticpolymers, having a strictly bimodal distribution of the rubberyparticles, which do not have the typical drawbacks of the products ofthe known art, which can be obtained with standard productionconfigurations and which have excellent physico-mechanical properties,mainly in terms of gloss and impact resistance.

An object of the present invention therefore relates torubber-reinforced vinyl aromatic (co)polymers, having a strictly bimodalmorphology, which consist of from 55 to 90% by weight of rigid polymericmatrix and from 10 to 45% by weight of a rubbery phase dispersed insidesaid rigid polymeric matrix in the form of grafted and occludedparticles and wherein said rubber particles consist of from 60 to 99% byweight, preferably 70-95%, of particles with a capsule or “core-shell”morphology and from 1 to 40% by weight, preferably 5-30%, of particleswith a “salami” morphology, said percentages being measured on the basisof the weight of the rubber particles only.

The term “vinyl aromatic (co)polymer”, as used in the presentdescription and claims, essentially refers to a product obtained fromthe polymerization of at least one monomer having the following generalformula:

wherein R is a hydrogen or a methyl group, n is zero or an integerranging from 1 and 5 and Y is a halogen such as chlorine or bromine, oran alkyl or alkoxyl radical having from 1 to 4 carbon atoms.

Examples of vinyl aromatic monomers having the above general formulaare: styrene, α-methyl styrene, methyl styrene, ethyl styrene, butylstyrene, dimethyl styrene, mono-, di-, tri-, tetra- and penta-chlorostyrene, bromo styrene, methoxy styrene, acetoxy styrene, etc. Styreneand α-methyl styrene are preferred vinyl aromatic monomers.

The vinyl aromatic monomers having general formula (I) can be used aloneor blended with other monomers which can co-polymerize. The amount ofcopolymerizable monomer can be up to 40% by weight, generally from 15 to35%, with respect to the total mixture of monomers. Examples of saidmonomers are (meth) acrylic acid, C₁-C₄ alkyl esters of (meth)acrylicacid, such as methyl acrylate, methyl methacrylate, ethyl acrylate,ethyl methacrylate, isopropyl acrylate, butyl acrylate, amides andnitriles of (meth)acrylic acid such as acrylamide, methacrylamide,acrylonitrile, methacrylonitrile, butadiene, ethylene, divinyl benzene,maleic anhydride, etc. Preferred monomers which can co-polymerize areacrylonitrile and methyl methacrylate.

According to the present invention, the core-shell particles have anaverage diameter of between 0.10 and 0.30 μm, preferably between 0.15and 0.25 μm, whereas the particles with a “salami” structure have anaverage diameter of between 1 and 5 μm, preferably between 2 and 4 μm.The diameter (D_(v)) of the particles was measured by means of thefollowing general formula:

D _(v) =ΣN _(i)(D _(i))⁴ /ΣN _(i)(D _(i))³

wherein N_(i) and D_(i) represent the number N_(i) of particles havingthe diameter D_(i).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transmission electronic microscope (T.E.M.) analysis of thepolymer of Example 1 (Reference) which shows a rubbery phase with“capsule” or “core-shell” particles of 0.2 μm.

FIG. 2 is a transmission electronic microscope (T.E.M.) analysis of thepolymer of Example 2 which shows a rubbery phase with a “strictlybimodal” distribution including “capsule” or “core-shell” particles of0.2 μm and “salami” particles of 2.0 μm.

FIG. 3 is a transmission electronic microscope (T.E.M.) analysis of thepolymer of Example 3 which shows a rubbery phase with a “strictlybimodal” distribution including “capsule” or “core-shell” particles of0.2 μm and “salami” particles of 2.2 μm.

Elastomeric products capable of supplying a rubbery phase dispersed inthe rigid polymeric matrix in the form of grafted and occluded particleswith a capsule or “core-shell” morphology, are selected fromhomopolymers and copolymers of 1,3-alkadienes containing 40-100% byweight of 1,3-alkadiene monomer, for example 1,3-butadiene, and 0-60% byweight of one or more mono-ethylenically unsaturated monomers selectedfrom styrene, acrylonitrile, α-methyl styrene, methyl methacrylate,ethyl acrylate.

Examples of 1,3-alkadienes copolymers are styrene-butadiene blockcopolymers, such as linear di-block rubbers of the S-B type, wherein Srepresents a polystyrene block having an average molecular weight Mwbetween 5,000 and 80,000, whereas B represents a polybutadiene blockwith an average molecular weight Mw between 2,000 and 250,000. In theserubbers the amount of S block ranges from 10 to 50% by weight withrespect to the total S-B rubber. The preferred product is astyrene-butadiene block copolymer having a styrene content equal to 40%by weight and viscosity in solution, measured at 23° C. in a solution of5% styrene by weight, ranging from 35 to 50 cPs.

Elastomeric products capable of providing a rubbery phase dispersed inthe rigid polymeric matrix in the form of grafted and occluded particleswith a “salami” morphology, are selected from homopolymers andcopolymers of olefins or 1,3 alkadienes incompatible with theelastomeric products which produce the capsule rubbery phase. Thecriterion for choosing said incompatible elastomers is that thedifference between the solubility parameter (δ), according toHildebrand, of the elastomer which produces the “capsule rubberyparticles and the solubility parameter, again according to Hildebrand,of the elastomer which produces the “salami” rubbery particles, ishigher than or equal to 0.5. Information on the solubility parameter canbe found in “CRC Handbook of Polymer-Liquid Interaction Parameters andSolubility Parameters”—Allan F. M. Barton—CRC Press Boca Raton, Boston.

Consequently, if a 40/60 styrene-butadiene copolymer (δ=8.7) is used forobtaining “capsule” particles, elastomers suitable for obtaining“salami” rubbery particles are polyisobutene (δ=7.9),polyisoprene-co-isobutene or butyl rubber (δ=7.8), polyisoprene (δ=8.2),EPDM rubber (δ=8.05). The preferred product is polyisoprene with asolution viscosity, measured as per above, of between 100 and 1000 cPs.

Conventional additives, generally used with traditional vinyl aromatic(co)polymers, such as pigments, stabilizing agents, plasticizers, flameretardants, antistatic agents, mold releasing agents, etc. can be addedto the rubber-reinforced (co)polymers, object of the present invention.

A further object of the present invention relates to a continuous-massprocess for the preparation of rubber-reinforced vinyl aromatic(co)polymers, with a strictly bimodal morphology, consisting of from 55to 90% by weight of rigid polymeric matrix and from 10 to 45% by weightof a dispersed rubbery phase inside said rigid polymeric matrix, in theform of grafted and occluded particles, wherein said rubber particlesconsist of from 60 to 99% by weight of particles with a “capsule” or“core-shell” morphology and from 1 to 40% by weight, preferably 5-30%,of particles with a “salami” morphology, said process comprising:

-   -   i) dissolving from 3 to 20% by weight of a rubber selected from        homopolymers and copolymers of 1,3-alkadienes containing 40-100%        by weight of 1,3-alkadiene monomer and 0-60% by weight of one or        more mono-ethylenically unsaturated monomers, having a        solubility parameter (δ₁) and from 0.05 to 8.0% by weight of a        rubber selected from homopolymers and copolymers of olefins or        1,3-alkadienes incompatible with the previous rubber, having a        solubility parameter (δ₂) which is such that δ₁−δ₂≧0.5, in a        liquid essentially consisting of at least one vinyl aromatic        monomer;    -   ii) polymerizing the resulting solution at a temperature ranging        from 50 to 250° C. optionally in the presence of polymerization        initiators and/or chain transfer agents;    -   iii) recovering the vinyl aromatic (co)polymer thus obtained.

The process object of the present invention can be carried out incontinuous using the equipment normally used for preparing traditionalreinforced vinyl aromatic (co)polymers, such as PFR plug flow reactorsor CFSTR reactors whose operating conditions are described, for example,in U.S. Pat. Nos. 2,727,884 or 3,903,202.

The rubbers are dissolved in the monomers possibly in the presence of aninert solvent in quantities ranging from 5 to 20% by weight with respectto the total. Examples of inert solvents which can be used in theprocess object of the present invention include aromatic hydrocarbonswhich are liquid at the polymerization temperature, such as, forexample, toluene, ethyl benzene, xylenes, or mixtures thereof. Thedissolution of the rubbers in the mixture of monomers and possiblesolvent is carried out in a mixer maintained at a temperature not higherthan 100° C.

The reactors are maintained, during the polymerization reaction, at apressure higher than the pressure at which the components fed evaporate.The pressure normally ranges from 0.5 to 5 bar whereas the temperaturepreferably ranges from 70 to 150° C. When PFR reactors are used, thetemperature is distributed in order to have two or more zones heated atdifferent temperatures.

The initiators used are of the conventional type adopted for thepolymerization of styrene, such as, for example, organic peroxyradicalic initiators. Examples of said initiators are: dibenzoylperoxide, tert-butyl peroctoate, tert-butyl perbenzoate, di-tert-butylperoxide, 1,1′-di-tert-butyl peroxy-3,3,5-trimethyl cyclohexane,1,1′-di-tert-butyl peroxy cyclohexane, etc. These initiators are addedin quantities ranging from 0.005 to 0.5% by weight with respect to themonomer.

The chain transfer agents are also those conventionally used in styrenepolymerization and are selected from mercaptans such as, for example,n-dodecyl mercaptan, t-dodecyl mercaptan (TDM), lauryl mercaptan,stearyl mercaptan, benzyl mercaptan cyclohexyl mercaptan, etc. Thesechain transfer agents are added in quantities ranging from 0.005 to 0.5%by weight with respect to the monomer.

Once the polymerization is finished, after reaching the desiredconversion degree (65-95%), the possible solvents present and thenon-reacted monomers are removed under vacuum and at a high temperature(200-260° C.), whereas the resulting polymer is extruded, cooled and cutinto pellets of the desired dimensions. The gaseous products which havebeen removed are condensed and possibly recycled.

As an alternative, the process, object of the present invention can becarried out in a completely equivalent manner, by means of a batchprocess in mass-suspension, using stirred autoclaves of thebatch-reactor type.

A second further object of the present invention therefore relates to amass-suspension process for the preparation of rubber-reinforced vinylaromatic (co)polymers having strictly bimodal morphology, consisting offrom 55 to 90% by weight of a rigid polymeric matrix and from 10 to 45%by weight of a rubbery phase dispersed inside said rigid polymericmatrix in the form of grafted and occluded particles, and wherein saidrubber particles consist of 60 to 99% by weight of particles with acapsule or “core-shell” morphology and from 1 to 40% by weight,preferably 5-30%, of particles with a “salami” morphology, said processincluding:

-   -   i) dissolving from 3 to 20% by weight of a rubber selected from        homopolymers and copolymers of 1,3-alkadienes containing 40-100%        by weight of 1,3-alkadiene monomer and 0-60% by weight of one or        more mono-ethylenically unsaturated monomers, having a        solubility parameter (δ₁), and from 0.05 to 8.0% by weight of a        rubber selected from homopolymers and copolymers of olefins or        1,3-alkadienes incompatible with the previous rubber, having a        solubility parameter (δ₂) which is such that δ₁−δ₂≧0.5, in a        liquid essentially consisting of at least one vinyl aromatic        monomer;    -   ii) pre-polymerizing the resulting solution at a temperature        ranging from 50 to 250° C. possibly in the presence of        polymerization initiators and/or chain transfers, until phase        inversion takes place;    -   iii) completing the polymerization in aqueous phase in the        presence of suspending agents.

In this type of process, the rubbers selected from those previouslyindicated, are dissolved in the monomers possibly in the presence of aninert solvent, in quantities ranging from 5 to 20% by weight withrespect to the total. Examples of inert solvents which can be used inthe process object of the present invention are aromatic hydrocarbonswhich are liquid at the polymerization temperature, such as, forexample, toluene, ethyl benzene, xylenes, or mixtures thereof. Thedissolution of the rubbers in the monomer mixture and possible solvent,is carried out in the same pre-polymerization autoclave (batch reactor)maintained at a temperature not higher than 100° C.

During the pre-polymerization reaction, the reactor is maintained at apressure higher than that at which the components fed evaporate.Normally the pressure ranges from 0.5 to 5 bar, whereas the temperatureis preferably between 70 and 150° C., with a stirring rate of between 10and 100 rpm. The initiators used are those conventionally adopted in thepolymerization styrene of styrene such as, for example, the organicperoxide radical initiators previously cited. Examples of theseinitiators are: dibenzoyl peroxide, tert-butyl peroctoate, tert-butylperbenzoate, di-tert-butyl peroxide, 1,1′-di-tert-butylperoxy-3,3,5-trimethyl cyclohexane, 1,1′-di-tert-butyl-peroxycyclohexane, etc. These initiators are added in amounts ranging from0.005 to 0.5% by weight with respect to the monomer.

The chain transfer agents are also those conventionally used in thepolymerization of styrene, cited above. Examples of chain transferagents are selected from mercaptans such as, for example, n-dodecylmercaptan, t-dodecyl mercaptan (TDM), lauryl mercaptan, stearylmercaptan, benzyl mercaptan, cyclohexyl mercaptan, etc. These chaintransfer agents are added in quantities ranging from 0.005 to 0.5% byweight with respect to the monomer.

Once the pre-polymerization with phase inversion has been carried out,the polymer is transferred to a second autoclave of the batch type, itis suspended in an aqueous phase (water/organic phase weight ratio ofbetween 1/1 and 3/2), containing one or more suspending agents, forexample sodium chloride, sodium naphthalene sulfonate and/orpoly-[(acrylic acid)-co-(2-ethyl-hexyl-acrylate)], possible peroxyinitiators or mercaptan chain transfer agents are added and thepolymerization is completed, by heating to temperatures of between 100and 170° C., until a full conversion of monomers to polymer is reached.At the end, the polymer is recovered with traditional methods.

Some illustrative examples are provided hereunder for a betterunderstanding of the present invention but in no way limit the scope ofthe invention itself.

EXAMPLE 1 Reference

4.2 kg of BUNA BL 6533 TC (BAYER) styrene-butadiene 40/60 copolymer,0.90 kg of PRIMOL 352 (ESSO) vaseline oil and 30 g of ANOX PP 18antioxidant in 24.9 kg of styrene monomer are dissolved in a 50 1 batchautoclave with an anchor stirrer, stirring for 5 hours at 85° C. 24 g ofTDM chain transfer agent are then added and the pre-polymerization iscarried out with grafting and phase inversion, heating and stirring thesolution thus obtained for 5 hours and 30 minutes at 120° C. Two 3 gdoses of TDM are added during the pre-polymerization, 3 hours and 5hours after the beginning of the heating to 120° C. In the end, thepre-polymer is transferred to a second 100 1 autoclave a with helixstirrer and is suspended in an aqueous phase (water/organic matterratio=1/1) containing NaCl (0.11% by weight), sodium naphthalenesulfonate (0.31% by weight) and poly-[(acrylicacid)-co-(2-ethyl-hexylacrylate)] (0.13% by weight). 30 g ofdi-tert-butyl peroxide are added and the polymerization is carried outuntil the total conversion of the monomer and cross-linking of therubbery phase, by heating under stirring for 1 hour at 120° C., 2 hoursat 140° C. and 3 hours at 155° C. In the end the polymer in the form ofbeads is washed, dried and pelletized in an extruder. Analysis of thepolymer with a transmission electronic microscope (TEM) shows a rubberyphase with “capsule” or “core-shell” particles of 0.2 μm (FIG. 1). Thephysico-mechanical properties on injection test samples of the polymerobtained are shown in table 1.

EXAMPLE 2

Example 1 is repeated with the only difference that instead of the BUNABL 6533 TC copolymer alone, a blend is used consisting of 3.6 kg of BUNABL 6533 TC copolymer and of 0.6 kg of polyisoprene IR 2200 L (NIPPONZEON).

Analysis of the polymer using a transmission electronic microscope (TEM)shows a rubbery phase with a “strictly bimodal” distribution including“capsule” or “core-shell” particles of 0.20 μm and “salami” particles of2.0 μm (FIG. 2). The physico-mechanical properties on injection testsamples of the polymer obtained are shown in table 1.

EXAMPLE 3

Example 2 is repeated, with the only difference that a blend consistingof 3.0 kg of BUNA BL 6533 TC copolymer and of 1.2 kg of polyisoprene IR2200 L (NIPPON ZEON) is used.

Analysis of the polymer using a transmission electronic microscope (TEM)shows a rubbery phase with a “strictly bimodal” distribution including“capsule” or “core-shell” particles of 0.20 μm and “salami” particles of2.2 μm (FIG. 3). The physico-mechanical properties on injection testsamples of the polymer obtained are shown in table 1.

TABLE 1 PROPERTIES UNIT EX. 1 EX. 2 EX. 3 MFI (200° C. - 5 KG) ISO 1133g/10′ 7.0 9.9 9.9 VICAT 5 KG ISO 306 ° C. 88.2 87.6 87.8 IZOD ASTM D 256½ * ½ int J/m 53 65 92 IZOD ASTM D 256 ½ * ⅛ int J/m 60 87 121 IZOD ISO180/1A int. KJ/m² 4.8 4.9 8.2 CHARPY ISO 179/1A int. KJ/m² 4.3 4.4 6.5GLOSS (20°) ASTM D 526 % 71 29 13 GLOSS (60°) ASTM D 526 % 96 77 61TENSILE STRENGTH ISO 527 σ_(S) MPa 30.3 26.6 25.6 σ_(R) MPa 23.2 19.919.4 ε_(S) % 20.6 21.1 28.4 ELASTIC MODULUS MPa 1950 1850 1890 FLEXURALSTRENGTH ISO 178 σ_(MAX) MPa 48.2 42.5 39.0 ELASTIC MODULUS MPa 20901990 1940 BALL DROP ISO 6603/2 2 mm J 1.8 15.0 14.7 BALL DROP ISO 6603/23 mm J 7.6 22.8 21.2

1-17. (canceled)
 18. A reinforced vinyl aromatic (co)polymer, consistingof: from 55 to 90% by weight of a rigid polymeric matrix and from 10 to45% by weight of grafted and occluded rubber particles dispersed insidethe rigid polymeric matrix; wherein the grafted and occluded rubberparticles have a strictly bimodal morphology consisting of from 60 to99% by weight of particles with a capsule or core-shell morphology andfrom 1 to 40% by weight of particles with a salami morphology, saidpercentages being measured on the basis of the weight of the rubberparticles only; and wherein the difference between the solubilityparameter (δ₁) according to Hildebrand of the elastomer which producesthe capsule or core-shell morphology of the rubbery particles and thesolubility parameter (δ₂) according to Hildebrand of the elastomer whichproduces the salami morphology of the rubbery particles, is higher thanor equal to 0.5 (δ₁−δ₁≧0.5).
 19. The reinforced vinyl aromatic(co)polymer according to claim 18, wherein the particles with capsule orcore-shell morphology have an average diameter ranging from 0.10 to 0.30μm, and the particles with a salami morphology have an average diameterranging from 1 to 5 μm.
 20. The reinforced vinyl aromatic (co)polymeraccording to claim 18, wherein the grafted and occluded particles with acapsule or core-shell morphology are elastomers selected fromhomopolymers and copolymers of olefins or 1,3-alkadienes containing40-100% by weight of 1,3-alkadiene monomer and 0-60% by weight of one ormore mono-ethylenically unsaturated monomers.
 21. The reinforced vinylaromatic (co)polymer according to claim 20, wherein the elastomersproducing the particles with a capsule or core-shell morphology arecopolymers selected from linear diblock rubbers of the S-B type, whereinS represents a polystyrene block having an average molecular weight Mwbetween 5,000 and 80,000, and B represents a poly-butadiene block withan average molecular weight Mw between 2,000 and 250,000.
 22. Thereinforced vinyl aromatic (co)polymer according to claim 21, wherein theamount of the S block ranges from 10 to 50% by weight with respect tothe total S-B rubber.
 23. The reinforced vinyl aromatic (co)polymeraccording to claim 22, wherein the elastomers producing the particleswith a capsule or core-shell morphology are copolymers selected fromstyrene-butadiene block copolymers having a styrene content equal to 40%by weight and a viscosity in solution, measured at 23° C. in a 5% byweight styrene solution, ranging from 35 to 50 cPs.
 24. The reinforcedvinyl aromatic (co)polymer according to claim 18, wherein the elastomersproducing the particles with a salami morphology are selected fromhomopolymers and copolymers of olefins or 1,3 alkadienes incompatiblewith the homopolymers or copolymers which produce the particles with thecapsule or core-shell morphology.
 25. The reinforced vinyl aromatic(co)polymer according to claim 24, wherein the elastomers producing theparticles with a salami morphology are selected from polyisoprenes witha viscosity in solution, measured at 23° C. in a 5% by weight styrenesolution, ranging from 100 to 1000 cPs.
 26. A rubber-reinforced vinylaromatic (co)polymer consisting of: from 55 to 90% by weight of rigidpolymeric matrix and from 10 to 45% by weight of grafted and occludedparticles having a strictly bimodal morphology consisting of: from 60 to99% by weight of particles with a capsule or core-shell morphology andfrom 1 to 40% by weight of particles with a salami morphology; which ismade by a mass-continuous process comprising: a. preparing a solutioncomprising: from 3 to 20% by weight of a rubber having a solubilityparameter (δ₁) selected from the group consisting of homopolymers andcopolymers of 1,3-alkadienes containing 40-100% by weight of1,3-alkadiene monomer and 0-60% by weight of one or moremono-ethylenically unsaturated monomers, from 0.05 to 8.0% by weight ofa rubber having a solubility parameter (δ₂) selected from the groupconsisting of homopolymers and copolymers of olefins or 1,3-alkadienes,and at least one vinyl aromatic monomer; b. polymerizing the solution ata temperature ranging from 50 to 250° C., optionally in the presence ofpolymerization initiators and/or chain transfer agents, to obtain arubber-reinforced vinyl aromatic (co)polymer with a strictly bimodalmorphology; and c. recovering the rubber-reinforced vinyl aromatic(co)polymer thus obtained; whereinδ₁−δ₂≧0.5.
 27. A rubber-reinforced vinyl aromatic (co)polymer obtainedby a mass-continuous process comprising: i) preparing a solutioncomprising: from 3 to 20% by weight of a rubber having a solubilityparameter (δ₁), selected from the group consisting of homopolymers andcopolymers of 1,3-alkadienes containing 40-100% by weight of1,3-alkadiene monomer and 0-60% by weight of one or moremono-ethylenically unsaturated monomers, from 0.05 to 8.0% by weight ofa rubber having a solubility parameter (δ₂) selected from the groupconsisting of homopolymers and copolymers of olefins or 1,3-alkadienesat least one vinyl aromatic monomer; ii) pre-polymerizing the solutionat a temperature ranging from 50 to 250° C. in the presence ofpolymerization initiators and/or chain transfer agents, until phaseinversion takes place; iii) after phase inversion, completingpolymerization in aqueous phase in the presence of suspending agents toobtain the rubber-reinforced vinyl aromatic (co)polymer with a strictlybimodal morphology; whereinδ₁−δ₂0.5, and the recovered rubber-reinforced vinyl aromatic (co)polymerconsists of: from 55 to 90% by weight of rigid polymeric matrix and from10 to 45% by weight of grafted and occluded particles having a strictlybimodal morphology, consisting of: from 60 to 99% by weight of particleswith a capsule or core-shell morphology and from 1 to 40% by weight ofparticles with a salami morphology.
 28. The reinforced vinyl aromatic(co)polymer according to claim 26, wherein the particles with acore-shell morphology have an average diameter ranging from 0.10 to 0.30μm, and the particles with a salami morphology have an average diameterranging from 1 to 5 μm.
 29. The reinforced vinyl aromatic (co)polymeraccording to claim 26, wherein the rubber having a solubility parameter(δ₁) is a linear diblock rubber of an S-B type, wherein S is apolystyrene block having an average molecular weight Mw between 5,000and 80,000, and B is a poly-butadiene block with an average molecularweight Mw between 2,000 and 250,000.
 30. The reinforced vinyl aromatic(co)polymer according to claim 29, wherein an amount of the polystyreneS block is from 10 to 50% by weight with respect to the totals weight ofthe S-B rubber.
 31. The rubber-reinforced vinyl aromatic (co)polymeraccording to claim 30, wherein the polystyrene S block content is 40% byweight and a viscosity in solution, measured at 23° C. in a 5% by weightstyrene solution, is from 35 to 50 cPs.
 32. The rubber-reinforced vinylaromatic (co)polymer according to claim 26, wherein the rubber having asolubility parameter (δ₂) is polyisoprene, and a viscosity in solution,measured at 23° C. in a 5% by weight styrene solution, is from 100 to1000 cPs.
 33. The rubber-reinforced vinyl aromatic (co)polymer accordingto claim 26, wherein wherein the vinyl aromatic monomer is representedby formula (I):

wherein R is a hydrogen or a methyl group, n is zero or an integerranging from 1 to 5 and Y is a halogen such as chlorine or bromine, oran alkyl or alkoxyl radical having from 1 to 4 carbon atoms.